Field devices are used in the process control and measurement industry to monitor and automatically control industrial and chemical processes. A field device transmits data representing a physical parameter such as temperature, pressure, position, or the like to a process control computer or host, and may also receive control signals that require the field device to take an action such as opening or closing a valve.
Field devices may be located in hazardous (classified) locations that can be a flammable or explosive environment. Field devices located in a flammable or explosive environment may be designed with intrinsic safety as a protection type. That is, the energy utilized by the device is low and incapable of causing a spark that may trigger an explosion or fire.
Field devices originally transmitted data to and from the host through a wired network. The wired network may also deliver power to the field devices. The power delivered by the wired network itself may be limited to an intrinsically-safe level when delivering power to field devices located in a hazardous (classified) location.
Wireless data transmission, however, is becoming popular. Wireless field devices include an antenna that sends and receives the wireless data. The antenna is housed in a radome conventionally made of an electrical insulator that separates the antenna from the ambient environment.
Wireless field devices may be designed from the start for wireless data transmission only, or may be converted from wired field devices utilizing a wireless network adapter that adds wireless capability to the wired field device.
Wireless field devices can be arranged in networks, such as mesh or star networks, which enable the use of relatively low power radio signals to and from the wireless field devices. Such low power wireless field devices are advantageous for use in hazardous (classified) locations.
Wireless field devices for use in hazardous (classified) locations typically utilize capacitors or other circuit elements in the antenna signal path. These circuit elements act as a high-pass filter that, in the event the antenna is shorted out, allows only a low-energy pulse to be generated and passed to the antenna. The low-energy pulse cannot trigger a spark.
A conventional wireless field device has a permanently fixed antenna. The antenna is not to be removed once the wireless field device is placed in the hazardous (classified) location.
It is desirable to have wireless field device that includes a replaceable or removable antenna, and particularly a wireless field device for use in a hazardous (classified) location that includes a replaceable or removable antenna. Such a wireless field device could have a defective or poorly performing antenna replaced in the field, even if the wireless field device were located in a hazardous (classified) location.
A concern of wireless field devices in a hazardous (classified) location is static electricity. Removing or attaching the antenna may itself generate a spark that jumps between the antenna radome (an insulator) and the device housing due to the accumulation of static electricity.
Thus there is a need for an improved wireless field device for use in a hazardous (classified) location with a removable antenna that resists sparking when removing or replacing the antenna.